Cryogenic containers, such as dewar type containers and cryogenic tanks, are used to store cryogenic fluids such as liquid nitrogen, oxygen, hydrogen and neon. A conventional cryogenic container includes an inner tank configured to contain the cryogenic fluid, and an outer tank configured to provide a thermal barrier between the cryogenic fluid and the environment. In addition, the outer tank forms an annulus around the inner tank in which insulation, and in some systems a vacuum, is contained. The outer tank and the annulus are constructed to minimize the conduction of thermal energy from the environment to the cryogenic fluid.
Cryogenic containers are commonly used by hospitals and in industrial applications where portability and compactness are not primary considerations. Cryogenic containers are also used in the transportation industry on ships and vehicles such as tank trucks and rail cars. In the transportation industry, portability and compactness are more of an issue, but in view of the scale of the vehicles, are not primary considerations.
Cryogenic containers are also used in alternative fueled vehicles (AFVs), such as cars and trucks, to store a cryogenic fluid for use as a combustion fuel for the vehicles. In this case, the cryogenic fluid can be in the form of liquid natural gas (LNG), compressed natural gas (CNG) or liquefied petroleum gas (LPG). The development of alternative fueled vehicles (AFVs) has been spurred by the Clean Air Act (1990) and the Energy Policy Act (1992). In addition, developing economies, such as China, have opted for polices which favor alternative fueled vehicles (AFVs) over conventional gas and diesel vehicles. With alternative fueled vehicles (AFVs), the portability and compactness of a cryogenic container can be a primary consideration. In addition, because the cryogenic liquids must be stored for periods of days or longer, these cryogenic containers must have a high thermal resistance from the environment to the cryogenic fluid.
Another technology that employs cryogenic containers is low temperature superconductivity. Superconductive materials have the ability to conduct electrical currents with no energy losses or resistive heating. In addition, superconductive materials exhibit magnetic properties that allow magnetic fields in excess of 20 tesla to be produced. Low temperature superconducting magnets are used in magnetic resonance imaging systems for medical applications, and in laboratories for experimental applications. In these applications, cryogenic containers are employed to maintain the low temperatures necessary for superconductivity.
Recently, superconductors, such as magnesium diboride (MgB2), have been discovered which exhibit superconductivity at temperatures approaching 40 K. Although this is a low temperature, it can be achieved using technologies that are less expensive than those used to achieve superconductivity in conventional superconductors, such as niobium alloys, which require temperatures of about 23° K.
In addition to their use in magnetic resonance imaging systems, superconducting magnets can be used for storing electrical energy. This technology is known as superconducting magnetic energy storage (SMES). For example, U.S. Pat. No. 5,146,383 to Logan entitled “Modular Superconducting Magnetic Energy Storage Inductor”, discloses a SMES system. U.S. Pat. No. 6,222,434 B1 to Nick entitled “Superconducting Toroidal Magnet System” also discloses a SMES system. In general, these prior art SMES systems are large non-portable, devices which are hundreds of feet in diameter. This technology, could be adapted to transportation and AFV industries, and to other applications as well, if the scale of the SMES system could be reduced.
The present invention is directed to all sizes of cryogenic containers, but particularly to a cryogenic container that is portable, yet has a low thermal conductivity, and a high thermal shielding capability. Further, the present invention is directed to a portable SMES system constructed using the cryogenic container. Still further, the present invention is directed to improved methods for shielding a cryogenic fluid from thermal energy.